It has previously been known to reinforce an acetal resin with glass fibers (see Japanese Patent Publication No. 28191/1969). Since, however, the acetal resin is chemically inert, mere mixing of the acetal resin with a reinforcing material such as glass fibers or glass powder does not produce a reinforcing effect, and in an extreme case, the mechanical strength of the resulting mixture becomes lower than the non-reinforced polyacetal resin.
To improve upon this point, methods have been proposed in which glass fibers are treated with aminosilane, vinylsilane or epoxysilane, and isocyanates, polycarbodiimide, alkoxymethylmelamines, etc. are used as coupling agents. Methods are also known to coat glass fibers themselves with a phenoxy resin, a polyamide, polyurea, polystyrene, etc. and to blend the coated glass fibers with an acetal resin.
These methods, however, have various defects in industrial practice. For example, they are expensive, the increase in mechanical strength is small, the heat stability of the resulting molding material is markedly impaired, and its toxicity is high.
Some of the present inventors previously found and proposed that the reinforcing effect of glass fibers or powder on an acetal resin can be greatly increased by incorporating a polyfunctional cyanate ester (or its prepolymer) or a mixture (or pre-reaction product) of a polyfunctional cyanate ester (or its prepolymer) and a polyfunctional maleimide (or its prepolymer) in the acetal resin (see Japanese Laid-Open Patent Publication No. 98,356/1983).